During a drilling operation on a conventional oil drilling platform, when the drill bit has penetrated such a distance into a borehole that only a small part of the drill string extends upwards from the upper surface of the drill floor, the drilling operation must be stopped, and a new tubular drill string section moved from a storage site or rack positioned outside the drill floor and connected to the upper end of the drill string. Once the new section is connected the drilling operation may be continued. Normally, the length of the drill string sections is 30 feet or about 10 m. This means that each time the drill bit has penetrated further 10 m into the underground the drilling operation has to be stopped and a further drill string section has to be added as described above.
This process creates significant idle time in which no actual drilling takes place. In view of the fact that the investment made in a drilling rig is very high (as an example the daily rent of an offshore rig may be on the order of U.S. $ 50,000) even a relatively small reduction of the necessary idle time is of great economical importance.
One solution commonly used to reduce the idle time on drilling rigs is to assemble two drill string sections, or singles, each having a length of about 10 m into a 20 m stand, or double, placing the singles in a mousehole adjacent to the drilling opening and connecting the singles by using air tuggers and spinning wrenches while the drilling operation proceeds. One exemplary system and apparatus for such offline standbuilding is described in U.S. Pat. No. 4,850,439, the disclosure of which is incorporated herein by reference. However, although these conventional offline standbuilding systems do create significant efficiencies in the drilling process, they generally utilize many complex pieces of equipment, such as, hoists and multi-purpose pipehandling machines that result in a system which is complicated, costly, and requires significant ongoing maintenance.
Accordingly, a need exists for a simpler, less costly system for providing offline stand building and pipehandling functionality to standard oil platforms.